Parasympathetic reinnervation will be investigated in the rat myocardium by measuring changes in distribution of one vagus nerve after a contralateral midcervical vagotomy. The distribution of vagal innervation will be measured by injecting (3H) proline into the dorsal motor nucleus and monitoring its axonal transport down the vagus and into the heart at varying times after either a unilateral vagotomy or a sham operation. Functional reinnervation will be investigated also by measuring the bradycardia produced reflexly with pressor responses to phenylephrine. Other factors possibly regulating reinnervation and to be investigated are the cholinergic muscarinic and nicotinic receptors. Reinnervation of the heart by vagal neurons will be followed after unilateral vagotomy in separate groups of atropine and chlorisondamine treated animals in order to determine if receptors influence the process of reinnervation. Control groups will be treated with vehicle. These studies are intended to provide information about the time course of vagal reinnervation in mammalian heart and investigate the role of cholinergic receptors in this process.